1. Field of the Invention
The present invention relates to optical disk apparatuses that can write to optical disks, i.e., recording media, including Compact Disk Recordable (CD-R) and Digital Versatile Disc Recordable (DVD-R), and in particular, relates to an optical disk apparatus that is designed to shorten the settling time when laser power changes from that for a write mode to that for a bias mode.
2. Description of the Related Art
As disclosed in page 5 and FIG. 1 of Japanese Unexamined Patent Application Publication No. 11-53753, in known CD-R drives and DVD-R drives, photoelectrically converted signals corresponding to laser diode light reflected from a disk are output from a photodetector integrated circuit and a front-monitor photodetector integrated circuit which are built into an optical pickup. The photoelectrically converted signals have a shape shown in FIG. 8A and are generally transmitted to a radio-frequency (RF) signal-processing integrated circuit through flexible cables.
The RF-signal-processing integrated circuit outputs, for example, servo signals and automatic-power-control (APC) signals based on the photoelectrically converted signals input through the flexible cables. To output servo signals, sampling the photoelectrically converted signals is required while the laser diode light is emitted at a bias laser power, i.e., while the photoelectrically converted signals are at a low level, as shown in FIG. 8A. When the recording speed increases, the sampling period shortens. Thus, the settling time of the photoelectrically converted signals needs to be shortened when the photoelectrically converted signals during a write period change to those during a bias period, i.e., while the photoelectrically converted signals are at a low level, as shown in FIG. 8A.
However, the above-described flexible cables electrically function as transmission lines, and possess frequency characteristics having peaks around 150 MHz. When the output slew rate of the front-monitor photodetector integrated circuit is enhanced to shorten the settling time in this situation, ringing 50 occurring in the photoelectrically converted signals is increased during the bias period, i.e., the period when the photoelectrically converted signals are at a low level (FIG. 8B). Thus, the settling time does not get shorter, but rather gets longer.
Since increasing the slew rate means that the load capacitance of, for example, capacitance components of the flexible cables and the input capacitance of the downstream RF-signal-processing integrated circuit must be charged for a short time, a large circuit current must run through the tail ends of the photodetector integrated circuit and the front-monitor photodetector integrated circuit. Accordingly, power consumption increases and exceeds an allowable power consumption of the package. Thus, the settling time cannot be shortened by enhancing the slew rate. Consequently, it is quite difficult to shorten the settling time when the write laser power changes to the bias laser power to less than 10 ns. When the recording speed increases, precision in servo control decreases, and tracking servo may even fail.
To solve the above problem, the photodetector integrated circuit and the front-monitor photodetector integrated circuit may be provided with sample-and-hold circuits for sampling and holding the photoelectrically converted signals to decrease the amplitude of the photoelectrically converted signals and to transmit these held signals. Such a decreased amplitude reduces ringing. On the other hand, rewritable media are written with a pulse train, and a plurality of quite short pulses occurs in the photoelectrically converted signals during the write period, as shown in FIG. 9. High level portions of these pulses are sampled to determine an optimum laser power by optimum power control (OPC) and to read out an absolute address. When the recording speed increases, the sampling timing shifts, thereby encountering difficulties in sampling the high level portions of the pulses. Sampling output is unstable, and thus OPC is unstable. Moreover, it is increasingly difficult to read out an accurate address, and thus the writing operation is unstable.
As described above, the sampled and held signals are preferably transmitted from the photodetector integrated circuit and the front-monitor photodetector integrated circuit to the RF-signal-processing integrated circuit which is located downstream. However, one type of RF-signal-processing integrated circuit can receive sampling output while another type can receive only differential output, these types having various kinds of interface. Thus, the versatility of the photodetector integrated circuit and the front-monitor photodetector integrated circuit which output the sampled and held signals is limited.